1. Field of the Invention
The present invention relates to a motorcycle having a main frame member, a fuel tank mounted on the main frame member, an ignition coil arranged below the fuel tank, and a coil-holding structure for holding the ignition coil. More particularly, the present invention relates to an arrangement of a coil-holding structure in a motorcycle for placing the ignition coils inconspicuously below the fuel tank without decreasing a capacity of the fuel tank, and which, during vehicle operation, facilitates dissipation of heat from the ignition coils.
2. Description of the Background Art
There are several known arrangements of disposing an ignition coil on a vehicle body of a motorcycle. During vehicle operation, the ignition coil applies a high voltage to an ignition plug of an engine of the motorcycle.
For example, the Japanese Patent Document JP-A-10-329772 discloses an ignition coil mounting structure for a motorcycle, in which an ignition coil is fixed to an inside portion of a lower cover, formed of a synthetic resin component, assembled in a leg shield of the motorcycle.
According to the ignition coil-holding structure as described in the Japanese Patent Document JP-A-10-329772, the ignition coil is fixed to the inside portion of the lower cover formed of the synthetic resin component. Hence, a heat-radiation effect of the ignition coil cannot be expected. The heat-radiation effect is particularly important with respect to an ignition coil of a high-output full-transistor-type ignition device since a primary coil thereof generates a large, unwanted heat.
Accordingly, it is necessary to take measures to radiate heat from the ignition coil. For enhancing a heat-radiation effect, it may be possible to adopt the structure which mounts a metallic stay on a frame body of the motorcycle, and fixes an iron core portion of the ignition coil to the stay by fastening bolts.
However, when the iron core portion of the ignition coil is mounted on the metallic stay, a closed magnetic circuit including the stay and the frame body is formed, and a secondary voltage of the ignition coil is lowered due to a core loss attributed to such a closed magnetic circuit. Accordingly, it is necessary to interpose a washer made of a non-magnetic material such as stainless steel between the iron core portion of the ignition coil and the metallic stay. Also, it is necessary to use bolts and nuts made of a non-magnetic material for fastening the ignition coil to the metallic stay.
In order to diffuse heat using the metallic stay, it may be possible to shorten a time for supplying electricity to a primary coil of the ignition coil. However, such a measure cannot increase a secondary voltage of the ignition coil. Accordingly, from a viewpoint of a demand for enhancing an air-fuel ratio and drivability of a motorcycle, it is not realistic to shorten the time for supplying electricity to the primary coil of the ignition coil.
Further sometimes, in a motorcycle, an ignition coil is arranged on a side surface of a main frame member. However, when a main frame member is formed by arranging a pair of sub-frame members in a vehicle width direction, such as in a case of a main frame member of a double cradle type frame which is often adopted by a custom-type motorcycle, a fuel tank is arranged astride the main frame member including the ignition coil which is mounted on a side surface of the main frame member. Hence, a large capacity of the fuel tank cannot be ensured.
The present invention has been made to overcome such drawbacks of existing arrangement of ignition coils. Accordingly, it is an object of the present invention to overcome the above-mentioned drawbacks of the related art, and to provide a motorcycle including the ignition-coil-holding structure which is suitable for enhancing a heat diffusing efficiency of an ignition coil without depending on the heat dissipation via a metallic stay.
Further, it is an object of the present invention to provide a motorcycle having a coil-holding structure which can arrange one or more ignition coils inconspicuously in appearance, and which can ensure a large capacity of a fuel tank mounted on a main frame member of the motorcycle.